This invention relates to metal matrix composites and more particularly to metal matrixes reinforced with ceramic particles.
There is a relentless search for easily produced, lightweight metallic materials having high strength and toughness. Alloys of magnesium-lithium and aluminum-lithium containing particulate or whisker reinforcement are under consideration for use in aerospace and other critical applications. Although these alloys already are attractive from a weight standpoint, it would be advantageous if their specific strength (modulus) could be improved conveniently by reinforcement. At the same time, it would be further beneficial if the property of elevated temperature superplasticity could be preserved in the composites. Superplasticity is the capability of certain polycrystalline materials to undergo extensive plastic deformation prior to failure. Various Mg/Li and Al/Li alloys possess this property, which is exploited in modern forming methods. A candidate material for particulate reinforcement of these alloys is boron carbide (B.sub.4 C, density=2.52 g/cc). The problem at hand is how this reinforcement material can be introduced in the fabrication process so as to reside uniformly and with proper interfacial characteristics in the final composition to preserve the desirable physical characteristics of the metal matrix material.
Discontinuous matrix metal composites are normally produced by the powder metallurgy method. Fine powders of the composite components (e.g. metals and reinforcement) in the correct concentration are mixed as thoroughly as possible. Blenders, shakers and liquid media are employed. Almost inevitably, the mixture is poor due to the difference in specific gravity, shape and size of the component particles. Moreover, during the consolidation step of the process, the thin oxidation coating on the metal particles presents a bonding problem. This layer must be broken by high compaction pressure to produce bonding. The hot pressing must be done often in vacuum to obtain good compaction and remove any hydration products which may be present. It, therefore, be highly advantageous if ingot metallurgy casting (IM) could be applied to the fabrication of these composites. In ingot casting, the metallic components, in larger sized pieces, are melted together. The reinforcement material is stirred in thoroughly and the mixture poured into a mold. This is a relatively simple and less costly procedure.
In the case of B.sub.4 C reinforcement addition to molten Mg--Li and Al--Li, the problem is twofold. The metals do not readily wet the particles and reaction is possible with too long an exposure at elevated temperature. The former characteristic renders uniform mixing all but impossible. The latter eventually allows reaction phases to form. In particular, the high temperature exposure eventually removes the carbon from the B.sub.4 C leaving behind boron which forms solid lumps in the melt due to its very high melting temperature (&gt;2000.degree. C.).